1. Field of the Invention
The present invention relates to a power train of an automatic transmission and, more particularly, to a power train of an automatic transmission that is capable of implementing eight forward gear stages and three reverse gear stages by using two composite planetary gear sets, two clutches and five brakes.
2. Description of the Related Art
In general, a power train of an automatic transmission is a device for shifting power generated from an engine by multiple stages by using hydraulic pressure to obtain a suitable rotatory force according to driving conditions.
The power train of an automatic transmission includes planetary gear sets for receiving power of the engine and varying it into a suitable rotatory force so as to output it, and engagement elements such as clutches or brakes for fixing rotational elements of the planetary gear sets, restraining rotational movement of them, or rotating them by virtue of power of the engine.
Typically, power trains of automatic transmissions provide four or five forward gear stages. Recently, six, seven, and even eight or more forward gear stages have been developed.
Such power trains typically engage three engagement elements for each gear state. This degrades hydraulic pressure control efficiency and is quite large structurally.